1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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24
25 #ifndef SHARE_OOPS_MARKWORD_HPP
26 #define SHARE_OOPS_MARKWORD_HPP
27
28 #include "metaprogramming/primitiveConversions.hpp"
29 #include "oops/compressedKlass.hpp"
30 #include "oops/oopsHierarchy.hpp"
31 #include "runtime/globals.hpp"
32 #include "utilities/globalDefinitions.hpp"
33
34 #include <type_traits>
35
36 // The markWord describes the header of an object.
37 //
38 // Bit-format of an object header (most significant first, big endian layout below):
39 //
40 // 32 bits:
41 // --------
42 // hash:25 ------------>| age:4 self-fwd:1 lock:2 (normal object)
43 //
44 // 64 bits:
45 // --------
46 // unused:25 hash:31 -->| unused_gap:1 age:4 self-fwd:1 lock:2 (normal object)
47 //
48 // 64 bits (with compact headers):
49 // -------------------------------
50 // nklass:32 hash:25 -->| unused_gap:1 age:4 self-fwded:1 lock:2 (normal object)
51 //
52 // - hash contains the identity hash value: largest value is
53 // 31 bits, see os::random(). Also, 64-bit vm's require
54 // a hash value no bigger than 32 bits because they will not
55 // properly generate a mask larger than that: see library_call.cpp
56 //
57 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
58 //
59 // [ptr | 00] locked ptr points to real header on stack (stack-locking in use)
60 // [header | 00] locked locked regular object header (fast-locking in use)
61 // [header | 01] unlocked regular object header
62 // [ptr | 10] monitor inflated lock (header is swapped out)
63 // [ptr | 11] marked used to mark an object
64 // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use)
65 //
66 // We assume that stack/thread pointers have the lowest two bits cleared.
67 //
68 // - INFLATING() is a distinguished markword value of all zeros that is
69 // used when inflating an existing stack-lock into an ObjectMonitor.
70 // See below for is_being_inflated() and INFLATING().
71
72 class BasicLock;
73 class ObjectMonitor;
74 class JavaThread;
75 class outputStream;
76
77 class markWord {
78 private:
79 uintptr_t _value;
80
81 public:
82 explicit markWord(uintptr_t value) : _value(value) {}
83
84 markWord() = default; // Doesn't initialize _value.
85
86 // It is critical for performance that this class be trivially
87 // destructable, copyable, and assignable.
88 ~markWord() = default;
89 markWord(const markWord&) = default;
90 markWord& operator=(const markWord&) = default;
91
92 static markWord from_pointer(void* ptr) {
93 return markWord((uintptr_t)ptr);
94 }
95 void* to_pointer() const {
96 return (void*)_value;
97 }
98
99 bool operator==(const markWord& other) const {
100 return _value == other._value;
101 }
102 bool operator!=(const markWord& other) const {
103 return !operator==(other);
104 }
105
106 // Conversion
107 uintptr_t value() const { return _value; }
108
109 // Constants
110 static const int age_bits = 4;
111 static const int lock_bits = 2;
112 static const int self_fwd_bits = 1;
113 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - self_fwd_bits;
114 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
115 static const int hash_bits_compact = hash_bits;
116 // Used only without compact headers.
117 static const int unused_gap_bits = LP64_ONLY(1) NOT_LP64(0);
118
119 static const int lock_shift = 0;
120 static const int self_fwd_shift = lock_shift + lock_bits;
121 static const int age_shift = self_fwd_shift + self_fwd_bits;
122 static const int hash_shift = age_shift + age_bits + unused_gap_bits;
123 static const int hash_shift_compact = 11;
124
125 static const uintptr_t lock_mask = right_n_bits(lock_bits);
126 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
127 static const uintptr_t self_fwd_mask = right_n_bits(self_fwd_bits);
128 static const uintptr_t self_fwd_mask_in_place = self_fwd_mask << self_fwd_shift;
129 static const uintptr_t age_mask = right_n_bits(age_bits);
130 static const uintptr_t age_mask_in_place = age_mask << age_shift;
131 static const uintptr_t hash_mask = right_n_bits(hash_bits);
132 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
133 static const uintptr_t hash_mask_compact = right_n_bits(hash_bits_compact);
134 static const uintptr_t hash_mask_compact_in_place = hash_mask_compact << hash_shift_compact;
135
136 #ifdef _LP64
137 // Used only with compact headers:
138 // We store nKlass in the upper 22 bits of the markword. When extracting, we need to read the upper
139 // 32 bits and rightshift by the lower 10 foreign bits.
140
141 // These are for loading the nKlass with a 32-bit load and subsequent masking of the lower
142 // shadow bits
143 static constexpr int klass_load_shift = 32;
144 static constexpr int klass_load_bits = 32;
145 static constexpr int klass_shadow_bits = 10;
146 static constexpr uintptr_t klass_shadow_mask = right_n_bits(klass_shadow_bits);
147 static constexpr uintptr_t klass_shadow_mask_inplace = klass_shadow_mask << klass_load_shift;
148
149 // These are for bit-precise extraction of the nKlass from the 64-bit Markword
150 static constexpr int klass_shift = 42;
151 static constexpr int klass_bits = 22;
152 static constexpr uintptr_t klass_mask = right_n_bits(klass_bits);
153 static constexpr uintptr_t klass_mask_in_place = klass_mask << klass_shift;
154 #endif
155
156
157 static const uintptr_t locked_value = 0;
158 static const uintptr_t unlocked_value = 1;
159 static const uintptr_t monitor_value = 2;
160 static const uintptr_t marked_value = 3;
161
162 static const uintptr_t no_hash = 0 ; // no hash value assigned
163 static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift;
164 static const uintptr_t no_lock_in_place = unlocked_value;
165
166 static const uint max_age = age_mask;
167
168 // Creates a markWord with all bits set to zero.
169 static markWord zero() { return markWord(uintptr_t(0)); }
170
171 // lock accessors (note that these assume lock_shift == 0)
172 bool is_locked() const {
173 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
174 }
175 bool is_unlocked() const {
176 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
177 }
178 bool is_marked() const {
179 return (mask_bits(value(), lock_mask_in_place) == marked_value);
180 }
181 bool is_forwarded() const {
182 // Returns true for normal forwarded (0b011) and self-forwarded (0b1xx).
183 return mask_bits(value(), lock_mask_in_place | self_fwd_mask_in_place) >= static_cast<intptr_t>(marked_value);
184 }
185 bool is_neutral() const { // Not locked, or marked - a "clean" neutral state
186 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
187 }
188
189 // Special temporary state of the markWord while being inflated.
190 // Code that looks at mark outside a lock need to take this into account.
191 bool is_being_inflated() const { return (value() == 0); }
192
193 // Distinguished markword value - used when inflating over
194 // an existing stack-lock. 0 indicates the markword is "BUSY".
195 // Lockword mutators that use a LD...CAS idiom should always
196 // check for and avoid overwriting a 0 value installed by some
197 // other thread. (They should spin or block instead. The 0 value
198 // is transient and *should* be short-lived).
199 // Fast-locking does not use INFLATING.
200 static markWord INFLATING() { return zero(); } // inflate-in-progress
201
202 // Should this header be preserved during GC?
203 bool must_be_preserved(const oopDesc* obj) const {
204 return (!is_unlocked() || !has_no_hash());
205 }
206
207 // WARNING: The following routines are used EXCLUSIVELY by
208 // synchronization functions. They are not really gc safe.
209 // They must get updated if markWord layout get changed.
210 markWord set_unlocked() const {
211 return markWord(value() | unlocked_value);
212 }
213 bool has_locker() const {
214 assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking");
215 return (value() & lock_mask_in_place) == locked_value;
216 }
217 BasicLock* locker() const {
218 assert(has_locker(), "check");
219 return (BasicLock*) value();
220 }
221
222 bool is_fast_locked() const {
223 assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking");
224 return (value() & lock_mask_in_place) == locked_value;
225 }
226 markWord set_fast_locked() const {
227 // Clear the lock_mask_in_place bits to set locked_value:
228 return markWord(value() & ~lock_mask_in_place);
229 }
230
231 bool has_monitor() const {
232 return ((value() & lock_mask_in_place) == monitor_value);
233 }
234 ObjectMonitor* monitor() const {
235 assert(has_monitor(), "check");
236 assert(LockingMode != LM_LIGHTWEIGHT, "Lightweight locking does not use markWord for monitors");
237 // Use xor instead of &~ to provide one extra tag-bit check.
238 return (ObjectMonitor*) (value() ^ monitor_value);
239 }
240 bool has_displaced_mark_helper() const {
241 intptr_t lockbits = value() & lock_mask_in_place;
242 return LockingMode == LM_LIGHTWEIGHT ? false // no displaced mark
243 : (lockbits & unlocked_value) == 0; // monitor | stack-locked?
244 }
245 markWord displaced_mark_helper() const;
246 void set_displaced_mark_helper(markWord m) const;
247 markWord copy_set_hash(intptr_t hash) const {
248 if (UseCompactObjectHeaders) {
249 uintptr_t tmp = value() & (~hash_mask_compact_in_place);
250 tmp |= ((hash & hash_mask_compact) << hash_shift_compact);
251 return markWord(tmp);
252 } else {
253 uintptr_t tmp = value() & (~hash_mask_in_place);
254 tmp |= ((hash & hash_mask) << hash_shift);
255 return markWord(tmp);
256 }
257 }
258 // it is only used to be stored into BasicLock as the
259 // indicator that the lock is using heavyweight monitor
260 static markWord unused_mark() {
261 return markWord(marked_value);
262 }
263 // the following two functions create the markWord to be
264 // stored into object header, it encodes monitor info
265 static markWord encode(BasicLock* lock) {
266 return from_pointer(lock);
267 }
268 static markWord encode(ObjectMonitor* monitor) {
269 assert(LockingMode != LM_LIGHTWEIGHT, "Lightweight locking does not use markWord for monitors");
270 uintptr_t tmp = (uintptr_t) monitor;
271 return markWord(tmp | monitor_value);
272 }
273
274 markWord set_has_monitor() const {
275 return markWord((value() & ~lock_mask_in_place) | monitor_value);
276 }
277
278 // used to encode pointers during GC
279 markWord clear_lock_bits() const { return markWord(value() & ~lock_mask_in_place); }
280
281 // age operations
282 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
283 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
284
285 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); }
286 markWord set_age(uint v) const {
287 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
288 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
289 }
290 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
291
292 // hash operations
293 intptr_t hash() const {
294 if (UseCompactObjectHeaders) {
295 return mask_bits(value() >> hash_shift_compact, hash_mask_compact);
296 } else {
297 return mask_bits(value() >> hash_shift, hash_mask);
298 }
299 }
300
301 bool has_no_hash() const {
302 return hash() == no_hash;
303 }
304
305 inline Klass* klass() const;
306 inline Klass* klass_or_null() const;
307 inline Klass* klass_without_asserts() const;
308 inline narrowKlass narrow_klass() const;
309 inline markWord set_narrow_klass(narrowKlass nklass) const;
310 inline markWord set_klass(Klass* klass) const;
311
312 // Prototype mark for initialization
313 static markWord prototype() {
314 return markWord( no_hash_in_place | no_lock_in_place );
315 }
316
317 // Debugging
318 void print_on(outputStream* st, bool print_monitor_info = true) const;
319
320 // Prepare address of oop for placement into mark
321 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
322
323 // Recover address of oop from encoded form used in mark
324 inline void* decode_pointer() { return (void*)clear_lock_bits().value(); }
325
326 inline bool self_forwarded() const {
327 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
328 return mask_bits(value(), self_fwd_mask_in_place) != 0;
329 }
330
331 inline markWord set_self_forwarded() const {
332 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
333 return markWord(value() | self_fwd_mask_in_place);
334 }
335
336 inline markWord unset_self_forwarded() const {
337 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
338 return markWord(value() & ~self_fwd_mask_in_place);
339 }
340 };
341
342 // Support atomic operations.
343 template<>
344 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
345 typedef markWord Value;
346 typedef uintptr_t Decayed;
347
348 static Decayed decay(const Value& x) { return x.value(); }
349 static Value recover(Decayed x) { return Value(x); }
350 };
351
352 #endif // SHARE_OOPS_MARKWORD_HPP